Punto 1.1 petrol (M plate)

My wife's car was acting really strange yesterday. Temperature setting was
jumping about all over the place. Would go away up to the red and then back
to normal in a matter of a few seconds. Even when it came to rest the needle
would be shaking away.  While this was happening the "injector light" would
occasionally come on and off.  Anyway we stopped the car (didn't switch off)
had a quick look under the bonnet but noticed nothing untoward.

Got a mile down the road to where we were going. Switched off the car. When
we got back in and tried to start it, it was dead. No engine turn over at
all. The battery is fine. Strange thing is when you turn the keys to the
ignition (without trying to start it) you can hear a clicking on and off
like a "relay" gone mad. This results in the ignition lights blinking on and
off in sync with the "relay" noise.

I've tracked the noise down to a wee box in the engine compartment located
at the back of the engine covered by a large hood. It is alongside two fuses
that seem unconnected with it. There is nothing in the Haynes regarding
this.

Anyone know what this unit is likely to be?
I thought at first it was something to do with her built in immobiliser as
she has had to use her red key a few times to get the car started but that
is now ruled out.

Thanx for any info

Steven.